Ion source



ION SOURCE I June 10, 1958 F. RAIBLE ETAL 2,838,676 v 1 Filed July 9.1953 5 Sheets-Sheet a FIG. 7

INVENTOR FRANK RAIBLE A ORNEY United States Patent ION SGURCE FrankRaible, Sands Point, Joseph E. Nemeth, Syosset, and Robert C. Nemeth,Huntington, N. Y., assignors to Vacuum-Electronics Engineering Co., NewHyde Park, N. Y., a partnership Application July 9, 1953, Serial No.367,040

Claims. Cl. 250-419 This invention relates to ion sources and moreparticularly to ion sources which may be employed to ionize themolecules of the constituent gases which are drawn into instruments forpurposes of analysis, as exemplified by ion sources employed in the massspectrometer. Still more particularly, the invention relates to methodsof operation of analytical instruments using electron beams as a meansof ionization and employing oil diffusion pumps as evacuating means indevices, such as a mass spectrometer for leak detection in which a probegas is employed.

Still more particularly, this invention relates to ion sources used indevices incorporating oil diffusion vacuum pumps generally referred toby K. C. D. Hickman in the articles entitled Vacuum pumps and pump oils,Journal of the Franklin Institute for February 1936, vol. 221, No. 2,page 215, and March 1936, vol. 221, No. 3, page 383, as well as inUnited States patents as follows: 1,857,506; 1,857,508; 2,080,421;2,147,488; 2,147,479, as examples disclosing various diffusion pumps andpump oils employed for evacuation in connection with which the ionsources of this application may be employed, it being understood thatthe oil diffusion pumps and pump oils so described are referred tomerely as examples.

Still more particularly, our invention relates to ion sources used indevices exemplified in the mass spectrometer in an arrangement describedin the publication Journal of Applied Physics, vol. 18, pages 3033,January 1947, an article entitled Mass spectrometer for leak detection.V r 1 It is known'to us that some ion sources have the inherent defectof being subject to'harmful contamination when used in spaces evacuated'by oil diffusion pumps, due to the formation of insulating depositswhich accumulate'on certain electrodes. These deposits progressivelycause a decrease in the-amount of the useful ions formed, due todistortions in the desired electrical field distribution withintheion'source itself. V

Known to us also is theexpedient attempting to correct this distortionin systems employing oil evacuating pumps byperiodically varyingthe-potential applied to the source Known. to us also are the efforts toinfluence the electron beam of the ion source by the influence of acollimating magnetic field in the ionization chamber. Such expedient isnot only unsuccessful but, at its best, is con- .finedto a sourceproviding a limited arrangement of the cooperating components ofelectrodes in relation to the deposition .ofthe deposits of insulatingmaterial.

In'each of the foregoing modes of operation, after a perio'd ofcontinued operation of such ion sources, the amount of contaminationreaches a degree whereit is impossibleor impractical evenv partially tooffset itseifect by the expedients described, aiming to correct thedistori 2,838,676 Patented June 10, 1958 tion. At such stage ofoperation, the quantity of ions formed is but a small fraction of thatwhich is required and which can only be obtained when the ion source isin its initial uncontaminated condition. Thereupon it is necessary tobreak the vacuum in the instrument in which the ion source is connected,to remove it from its enclosure and to disassemble its component parts.The contaminating deposits must then be removed from the ion source, theparts reassembled and the source reinstalled and again re-evacuated.This type of corrective procedure is extremely laborious,time-consuming, and results in taking the apparatus out of serviceduring the period for effecting the repairs. In addition, thecontaminating efiect progressively alters the sensitivity of the relatedapparatus and where the mechanism is employed in connection with amassspectrometer leak detector, or the like instrumen "in devices which areevacuated by oil diffusion pumps with all of the incident advantagesthereof.

Still more particularly it is an object of this invention to provide amode of operation of devices employing ion sources using oil diffusionpumps as the evacuating me dium which assures that the quantity of ionsformed by the ion source is substantially constant, to avoid diminishingsensitivity of the instrument employing the ion source and assuringconstant operation over extended periods of time without interruptionspreviously caused by the necessity to clean and remove the contaminatinginfluence of carbonaceous or like insulating deposits.

Still further it is an object of this invention 'to provide an ionsource which provides substantially a constant number of ions which maybe employed in measuring instruments without special attention oradjustment by reason of periodic changes in sensitivity which havecharacterized prior devices and thereby to assure continuous operationwith the minimum of care and attention.

Still further it is an object of our invention to provide an ionsourceand method of using the same in the presence of an oil diffusionpump characterized by passing the electron beam adjacent to a repellerfor the ions under conditions wherein the ion repeller or the like isheated to prevent insulating or deleterious deposits being formed.

To attain these objects and such further objects as may appear herein orbe hereinafter pointed out, We make reference to the accompanyingdrawings, forming a part hereof, in which Figure 1 is a wiring diagramand fragmentary section of an ion source illustrating our invention;

Figure 2 is a fragmentary, longitudinal section of the source end of atube employing our invention, taken along the line 2-2 of Figure 1;

Figure 3 is a fragmentary section taken on the line 33 of Figure 2;

Figure 4 is a wiring diagram and fragmentary section of anotherembodiment of our invention;

Figure 5 is a fragmentary, longitudinal section of the source endof atube taken on the line S5 of Figure 4;

Figure 6 is a section taken on the line 6-6 of Figure 5;

Figure 7 is a diagrammatic representation of a mass spectrometerincorporating the ion source.

Referring to the drawing, in Figure 1 there is shown a Wiring diagramand fragmentary section of an ion source which are the essential features of the source and of a tube in which 11 is a tungsten filamentarranged to be heated to form the cathode and which is supported onposts 12 and 13, which also serve as electrical condoctors to pass theheating current through the filament 11 so as to cause it to reach atemperature sufficiently high to emit electrons. Filament heatingcurrent is supplied to the filament 11 by the center-tapped filamenttransformer 14 through conductors 15 and 16, leading to the posts 12 and13. The posts 12 and 13 are sealed into the vacuum system by the seal 17Encasing the filament 11 is a shield box 18, having an open end 19 and aclosed end 20. The shield box serves to enclose the filament and postswhich are projected through the opening 19. The shield box 13 serves asthe anode, having a potential which is positive with respect to thefilament 11 so that the electrons emitted from the filament are directedto the closed end 20 of the shield box, which is positioned in closeproximity to the filament 11.

The closed end 20 is provided with a small slit or aperture 21 which isdirectly opposite the filament 11. Some of the electrons attractedtoward the closed end 20 pass through the slit 21 and enter the ionizingcharnber 22 where ionization of the gaseous molecules is effected bycollision of the electrons with the molecules.

The ionizing chamber 22 is bounded on its lower side by a shield plate23. The shield plate 23 and the shield box 18, by means of theconductors 24 and 25, respectively, are operated at approximately thesame potential. Shield plate 23 is provided with a beam slit 26 throughwhich the ions formed are expelled from the ion source.

In opposite position to the shield plate 23, around the path of theelectrons entering the chamber 22, there is provided the grid 27. Thisgrid consists of a fine heater wire or ribbon, such as Nichrome, whichis distributed along the path of the electrons in zig zag fashion in aplane parallel to the shield plate 23. The ionization chamber 22 isfurther defined by the end angle shield 28 and the side angle shields 29and 30. The end angle shield 28 and the side angle shields 29 and 30 arewelded to the shield plate 23 to be spaced from the grid 27 and toremain insulated from it.

The method of supporting the grid 27 and insulating the variouscomponents from it may be by well-known techniques, it being sufficientto state that the grid may be supported upon a sheet of mica, althoughother expedients will readily suggest themselves to the skilledtechnicians in this field. I Also, it is to be noted that the methods ofsealing the electrical leads into the vacuum systems are well known andare merely diagrammatically shown for an understanding of the invention.

Spaced from the shield plate 23 and in parallelism therewith is a focusplate 31 and .a source plate 32, having a source slit 33 for the ionbeam B. The assembly is held to a source flange 34 in spaced relation bya plurality of insulating spacers 35 through which the bolts 36 pass.These bolts are covered with insulation 36a along their entire length.The shield box 18 is supported from these bolts in spaced position fromthe focus plate 23 by means of a pair'of extended arms 18:: formed withears 18b, through which the bolts pass. Only a fragment of the sourceflange 34 is shown, as other details for connecting the assembly,including connections to power supply and internal leads, are suchwell-known techniques as not to require illustration. I The grid 27 isconnected by leads 37 and 38 to a direct current source of heatingcurrent 39, to heat the grid 27 to temperatures ranging up to 1800" F.The end of the grid 27 nearest the filament 11 is connected by the lead40 to the voltage divider 41, to which the lead 24 from the plate 23 islikewise connected. The

grid 27 is operated so as to be more positive in potential than theshield plate 23, by an amount diagrammatically identified as E Thecomponent E is adjusted so that the average potential of the grid 27 issufiiciently more positive than that of the shield plate 23 to repelpositive ions formed between the two toward the plate 23, and thenproject them out of the ion source through the slit 26. Because of thisrepelling action, grid 27 is hereinafter referred to as ion repellergrid.

In the construction as shown, the ion source is characterized byelectrons projecting into the chamber 22, so that they pass between theplate 23 and the ion repeller grid 27 which is operated preferably atelevated temperatures, thereby preventing contaminating insulatingmaterials from forming on the repeller surfaces of the wire, whenassociated with an oil diffusion pump to draw the ion and constituentgases into an atmosphere evacuated to a pressure of from 2 l0- to 10*millimetres of mercury. The result achieved is that when this ion sourceis operated continuously in conjunction with oil diffusion vacuum pumps,the number of ions in the ion beam remains constant and there is nonecessity for periodic dismantling of the assembly for cleaning.Likewise, frequent adjustment after the initial potentials have been setfor appropriate operation is avoided.

It is to be understood that in the wiring diagram given, the directcurrent sources 39, 43 and 44 have been indicated as batteries havingthe polarity as identified. In practice, rectifier power supply circuitshaving suitable filters are used. Similarly, the electron emission ofthe filament 11 is usually regulated by a suitable regulatory circuitwhich is only diagrammatically illustrated, since such circuits are wellknown to the skilled technician and need not be particularized to agreater degree.

Without intending to be bound by the explanation of what occurs, We havefound that the absence of a red hot to incandescent conditionparticularly as on the plate 23 does not upset the desired fielddistribution in the ion source because the electrons are directed nottoward plate 23 but to the grid 27. The electrons passing through thespace between the plate 23 and the grid 27 will have a tendency tostrike the heated surface because it has a potential which is morepositive and has an attraction for the electrons. The possibleaccumulation of a deposit on plate 23 appears to be immaterial.

While we have shown and described an ion repeller which is heated alongthe path of the electron beam to a temperature in its specific formabove the actual carbonization point of the oil used in the diffusionpump and in a range above the boiling point of the oil when used asdescribed, we may both heat the walls of the ionization chamber andcontour them to provide an accelerator of the electrons. This weaccomplish by surrounding the ionizing chamber with a coiled heaterwhich, with regard to its size and the number of turns as well as thespacing and potential, achieves a focusing effect upon the electronbeam.

For this purpose we now refer to the embodiment illustrated in Figures4, 5 and 6 in which the source end of the tube 10 is provided with thesame arrangement of filament 11 to be heated to form the cathode and issupported similarly on posts 12 and 13 to pass the heating currentthrough the filament 11. As in the prior embodiment the filament 11 isextended into the shield box 18, having an open end 19 and closed end20, the shield box serving as the anode. The shield box 18 has a pair ofextended supporting arms 18a formed with ears 18b. The closed end isprovided with an emission slit 21, as before, with its long dimension inparallelism with the filament 11 and in close adjacency to and oppositethe filament 11.

In this embodiment, however, surrounding the slit 21 and the path takenby the electrons passing through the slit is an ionizing chamber 22a.Sfilch chamber is defined by a helical coil 27a having longitudinallyspaced convolutions of rectangular section and which is heated toincandescence or red heat by means of an electric current supply bydirect current source 39a through the conductors 37a and 38a. Theelectrons passing through the slit 21 now enter the space 22a surroundedby coil grid 27a, where the accelerated electrons produce ions bycollision with the gaseous molecules, to one side of the end shield 28a.

The end of the coil grid 27a nearest the filament 11 is maintained at aslight positive potential Ea with respect to the anode by means of avoltage divider 41a. Thus, the field of the coil grid 27a penetratesthrough the slit 21 more effectively to draw electrons out into theionization space.

Shield plate 23a has a slit 26a closely adjacent to the coil grid 27aand slit 26a has its longest dimension in parallelism to thelongitudinal axis of the grid coil 2711. Plate 23a is operated at aslight negative potential with respect to the coiled grid 27a by meansof voltage divider 41b. The negative field of shield plate 23apenetrates into the coil grid 27a, thereby attracting the positive ionswhich are formed and causing them to be expelled from the source throughthe slit 26a.

Since the coil grid 27a is operated at incandescence or red heat, thesurface of the wire is prevented from being contaminated, with theresult that the ion source maintains constant the number of ions formed,fulfilling one of the objectives described in connection with the firstembodiment. However, this embodiment has the additional advantage thatby an appropriate design of the coil 27a as regards the size of theturns as well as the spacing and potential difference between turns, afocusing effect can be imparted to the electron beam within the coil27a. This focusing effect, which may be explained as due to theelectrostatic field distribution within the coil, influences the beam toprevent divergence from a path corresponding to theaxis of the coil 27a.The effect produced is to have more extensive collision between theelectrons and the molecules of the gaseous elements along the axis ofthe coil. The ions resulting from the collisions, since they aredirectly opposite the slit 26a, have the greatest probability of passingthrough the slit and being expelled from the source rather than strikingthe shield plate 23a. By providing an ionizing chamber which is heatedalong the entire path of the bombarding electrons and whichsimultaneously effects a focusing action of the electron beam, thenumber of useable ions produced by the source is greatly increased.

In Figure 7 there is shown a diagrammatic representation of a massspectrometer S used for leak detection, the same incorporating the ionsource above described and including filament 11 with leads 15, 16;shield plate 23 formed with beam slit 26; the grid 27; the focus plate31; and the source plate 32 formed with source slit 33 for passing theion beam B. The spectrometer further includes analyzer magnet M; acollector plate or target C with electrical leads therefrom forconnection to the usual amplifier and measuring devices, not shown; aninlet H for the incoming helium gas; and a diffusion pump P forevacuating the system. These instruments employ ion sources for theprimary purpose of ionizing helium which gas is used as the tracer orprobe gas in the detection of leaks. The helium ions formed in the ionsource are magnetically separated from other ions which are formed bymagnet M and they are caused to impinge upon conducting target orcollector C. Here they give up their charge, causing an electricalcurrent to flow, which is called the collector current.

The mechanism of mass spectrometer leak detection may be summarized asfollows:

The magnitude of the collector current is a measure of the number ofhelium ions formed which, in turn, is a measure of the quantity of thehelium entering the mass spectrometer which, in turn, is a measure ofthe quantity of helium passing through the leak being detected, andwhich in turn depends upon the size of the leak.

While we have shown and described the heating of the walls of anionization chamber by the provision of an ion repeller in the form of aheater filament which may be heated to elevated temperatures and whichmay be located to one side of the ionization chamber or to effectfocusing of the ion beam, it will be understood that our inventioncontemplates providing means to heat the effective components externallyapplied, such as by radiation, conduction or by bombardment by electronswhile applying potential difference in relation to other component partsto repel the ions,

While we have described as the preferable operating condition and as aseparate expedient, the application of heating for the grid 27 toachieve a condition of red heat or incandescence to insure against thedeposition of the diffusion pump oil and also any other vapors, separateheating means may be supplied to the grid 27, and coil 27a, with orwithout additional heat along the path of the electron beam passingthrough the ionization chamber, with due regard to the nature of thediffusion pump oil, and the constituents of gas mixture being ionized inthe use of the ion source. Thus we may use as a conditioning operationand employ as separate means to apply heat at a temperature sufiicientto prevent the condensation of the pump oil vapor on the surface of thewire and at least one of the walls defining the path for the electronbeam. The temperature contemplated may be within a range between theboiling point of the diffusion pump oil used, of about 250 F. and thatof incandescence of about 1800 F., the exact temperature beingdetermined by such factors as the constituents of the gas being ionized.Accordingly, in the claims where we use the expression of applyingdecarbonization temperatures or heating above the carbonization point,or the like terminology, we mean to include thereby the broader phasesof our invention encompassing temperatures which remove or prevent theformation of carbonization deposits, by volatilization or evaporation ofthe diffusion pump oil above its boiling point or conditions forseparately applying heat in the operations described which preventcondensation of the pump oil vapor and which, but for the heat appliedto remove or prevent condensation, would carbonize or form insulating orcontaminating deposits. The terms defined are accordingly used in thesense of preventing a condition of formation as well as of positiveremoval of the contaminating deposits.

Having thus described our invention and illustrated its use, what weclaim as new and desire to secure by Letters Patent, is:

1. In an ion source for purposes described, a cathode providing anelectron emitting source, an ionizing chamber providing an electronbeam, and heating means and electrical potential means therefor to theside of said chamber applying decarbonization temperatures and ionrepeller influence along the path of the electrons for the reduction ofinsulating deposits and providing a uniform source of useable ions.

2. An ion source for the purposes described having an electron emitterand heat source therefor and an ionizing chamber for projecting anelectron beam along a path through said chamber and heating meansindependent of said emitter and its heat source having decarbonizationtemperature effective along the, path of the bombarding electronspassing through said chamber, whereby the number of useable ionsproduced by the source is increased.

3. An ion source in accordance with claim 1 wherein said heating meansand electrical potential means therefor simultaneously impress anelectrical potential for directionally influencing the ions.

4. An ion source in accordance with claim 1 wherein said heating meanssimultaneously has means to impress an electrical potential fordirectionally repelling 76 ions.

A alsslie" length of the beam an ion repeller including means to heatthe chamber and ion 'repeller at a decarbonization temperaturedirectionally to influence the ions.

7 7. In an ion source for the, purposes described, a cathode providinganelectron emitting source, an anode surrounding said cathode and definingto one side thereof an ionization chamber, an ion repeller along thepath of the electron beam projected in said ionizing chamber, said ionrepeller comprisinga heater grid and including means to provide heatthereto attemperatures ranging up to about 1800 F.

8. In an ion source for the purposes described, a cat11- ode providingan electron emitting source, an anode surrounding said source and havingan outlet through which an electron beam may project, an ion repellerand shield plate along the path of the electron beam, the ion repellercomprising a heater grid including means for setting up a repellerpotential and hcating'the grid to elevated temperatures.

9. In an ion source comprising an anode, an ionization chamber and acathode spaced in relation to each other with means to provide an ionbeam, the combination therewith comprising an ion'repeller, togetherwith means to heat the repeller to elevated temperatures and connectedwith means for impressing a repeller potential.

10. In an ion source of the character described comprising an encasinganode, a cathode within said anode, an emission outlet through-saidanode for directing electrons into an ionization chamber for ionizationof gaseous molecules, the combination wherein said ionization chamberincludes ionbeam focusing means and ion repelling means provided withheater means to heat the ion repeller to elevated temperatures.

11. An ion source in accordance with claim 6 wherein said ion repellerincludes fine heater filament positioned in close zig zag fashion,including means to'heat the same to; elevated temperatures.

' 12.: An ion source in accordance with claim 1 wherein saidheater'means'includes a heater filament coiled about said ionizingchamber, including means to heat the same to temperatures rangingupwardly to about 1800 F, the coil comprising loops spaced from eachother and means to provide electrostatic fields between adjacent loopsof said coil. 7

13. In a massjspectrometer for leak detection having a spectrometer tubeincluding an oil diffusion pump as an evacuator and having an ionsource, which includes an electron generator, an ionization chamberthrough which an electron beam is passed and including an ion repeller,the combination therewith or" means to heat the chamber to'decarbonization temperatures along the path of the beam through saidchamber.

14. In a mass spectrometer for leak detection having a spectrometer tubeincluding an oil diffusion pump as an evacuator and having an ionsource, which includes an electron generator, an ionization chamberthrough which an electron beam is passed, the combination therewith ofmeans 'toi'heat the chamber to decarbonization temperatures along thepath of the beam through said chamber, and means to impress a potentialdifference on said heater means.

15. In a mass spectrometer for leak detection having a spectrometer tubeincluding an oil ditr'usion pump as an evacuator and having an ionsource, which includes an electron generator, an ionization chamberthrough which an electron beam is passed, the combination therewith ofmeans to heat the chamber to decarbonization temperatures along the pathof the beam through said chamber, and means to impress a potentialdifference on said heater means, said heater means comprising a coilhaving spaced convolutions and the potential diiierence being impressedbetween elements of said heater means.

References Cited in the file of this patent UNIT ED STATES PATENTS2,624,845 Thompson Jan. 6, 1953

